1. Field of the Invention
The present invention relates to a connection structure of plate type vertical drains and a circular horizontal drainpipe and a method of constructing a horizontal drain layer in soft ground using the same. More particularly, the present invention relates to a fitting type of connection structure of plate type vertical drains and a circular horizontal drainpipe and a method of constructing a horizontal drain layer in soft ground using the same, in which the plate type vertical drains are coupled to the circular horizontal drainpipe using cylindrical fitters, so that excess porewater can be easily drained from underground even if the horizontal drain layer is constructed by pouring soil instead of sand, thereby saving construction costs and reducing the construction period.
2. Description of the Related Art
Soft ground is generally composed of clay soil, located in reclaimed land, the beach, the riverside, sloughs, swamplands and so on, and does not provide a sufficient amount of support to serve as a foundation for a building. When a building is directly constructed on the soft ground, the foundation may collapse, or major consolidation and settlement may occur.
As an approach to improve the soft ground, there is proposed a vertical drain method, in which a plurality of vertical drains is artificially constructed in the soft clay ground in order to decrease the draining distance, thereby promoting the consolidation of the ground.
According to the conventional vertical drain method, a horizontal drain layer is formed on the top surface of the soft ground, followed by constructing plate type vertical drains in the soft ground, and then cover soil is laid thereon, so that excess porewater pressure corresponding to the load of the cover soil is created in the soft ground, thereby guiding excess porewater from underground to drain to sand of the horizontal drain layer through the plate type vertical drains.
Therefore, in order to drain porewater from the soft ground treatment consolidation method, it is important to ensure the constructability of the plate type vertical drains. However, the drainability of the horizontal drain layer is regarded as the most important factor.
In the conventional vertical drain method, as material providing the drainability of the horizontal drain layer, sand deposited on a river or sea bed or crushed sand is essentially selected.
That is, a geosynthetic mat is laid on the top surface of the soft ground in order to reinforce the ground for an embankment, and a sand mat for horizontal drainage is laid thereon at a thickness of about 50 to 100 cm, so that excess porewater from the plate type vertical drains can be guided to and drained through the sand mat by the hydraulic gradient.
However, when a large area of soft ground has to be reinforced, some of the plate type vertical drains are far away from the outer circumference of the sand mat, and consolidation settlement takes place. Restriction/deformation may occur in the cross section of the sand mat, or foreign materials (i.e. the organic soil) may penetrate into the sand mat, so that the sand mat may gradually lose its normal horizontal drainage capability.
This increases the resistance of the sand mat against excess porewater from underground, thereby producing side effects such as delayed consolidation due to degraded permeability and the behavior of residual porewater pressure. Accordingly, residual settlement becomes greater than estimated.
Furthermore, the settlement of the central area of the ground causes excess porewater to accumulate in the sand mat instead of being drained therethrough. Thus, sump pumps and pipelines are additionally required in order to forcibly drain accumulated porewater out of the central area.
In the vertical drain method for treating soft grounds, which uses sand for the horizontal drain layer, most of the sand mat of the horizontal drain layer is below ground water level. The sand mat is deformed by the penetration of foreign materials and the reduction of the cross section and thus its permeability is degraded, thereby causing delayed consolidation and residual consolidation in a great amount.
Furthermore, since the supply of natural sand is being gradually exhausted, the development of new sources of sand is required. In connection with this, however, environmental damage is expected, and the cost of sand is sharply rising, thereby increasing construction costs.
As an approach to overcome these problems, Korean Patent Publication No. 2000-0074033 proposes a method of reinforcing soft ground without the use of sand. According to this method, vertical drains are constructed in a soft ground, a horizontal drainpipe is connected to the vertical drains using T-shaped connectors, and a counterbalance process is performed thereon, so that excess porewater can be drained from underground without the use of sand.
In this conventional method, however, the T-shaped connectors are provided to pipes of the vertical drains, and each of the horizontal drainpipes is cut by a corresponding length. Accordingly, a worker has to calculate respective lengths and cut respective horizontal drainpipes based upon the calculated lengths.
Since each of the cut horizontal drainpipes has to be inserted into a corresponding one of the T-shaped connectors, a large number of workers is also required. This acts as a factor that increases construction costs, possibly prolonging the construction period.
Furthermore, since only an end portion of the horizontal drainpipe is inserted into the T-shaped connector, the horizontal drainpipe may be easily removed from the T-shaped connector in the event of settlement of the soft ground. This may lead to a connection defect, thereby degrading the ability to drain excess porewater.
Japanese Patent Publication No. 1992-254610 also discloses a method of improving soft ground, by which a horizontal drainpipe is directly connected to a vertical drainpipe to drain excess porewater from underground. However, construction thereof is difficult since a welding process or an auxiliary drain is required to directly connect the horizontal drainpipe to the vertical drainpipe.
Since the horizontal drainpipe, made of steel is directly connected to the vertical drainpipe, the horizontal drainpipe and the vertical drainpipe may be disconnected from each other, or may be broken in the worse case, thereby hindering drainage. In some cases, additional construction for repair is subsequently required.
Furthermore, Korean Patent No. 0390285 discloses a method of promoting dewatering of soft ground using a combination of a cylindrical drain and a plate type drain. In this method, a cylindrical drain having a filtering corrugated structure is constructed in an upper level area of a soft ground, and a plate type drain is constructed in a lower level area of the soft ground, and is coupled to the cylindrical drain using a coupling, and a Styrofoam mat is used in place of sand.
In this conventional method, however, since the cylindrical drain is coupled, via the coupling, to the top portion of the plate type drain underground, constructability is degraded. Furthermore, since a band strap is used to bind the drains, the coupling may be easily loosened.
Moreover, the Styrofoam mat used as a horizontal drain layer makes it difficult to flexibly cope with the settlement of the soft ground, and additional materials such as an EPS mat and nonwoven cloth, laid on the top of the Styrofoam mat, complicate construction and may also raise overall costs.